1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. In this specification, the semiconductor device refers to any device which can function with the use of semiconductor characteristics.
2. Description of the Related Art
In recent years, the information society has been increasingly developed, and the demand for higher speed, larger capacitance, smaller size, lighter weight, or the like of information communication equipment such as a personal computer or a mobile phone has been increased. With such a trend of the times, LSI (large scale integration) has been needed to be provided with higher integration, higher speed, and lower power consumption, and as a result, higher performance and miniaturization of each transistor for forming LSI are essential.
In view of higher performance and miniaturization of a transistor, various structures have been considered for a thin film transistor. For example, in order to realize higher performance and miniaturization of a thin film transistor, reduction in thickness of a semiconductor layer has been progressed.
For example, the present applicant has proposed to use a thin crystalline semiconductor film with a thickness of less than or equal to 30 nm as an active layer of a TFT. Specifically, a technique has been disclosed in which, after an amorphous semiconductor film with a thickness of greater than or equal to 40 nm is crystallized, the crystallized semiconductor film is entirely or selectively etched to form a region with a thickness of less than or equal to 30 nm, and the region thinned to less than or equal to 30 nm is used as a channel formation region (Reference 1: Japanese Published Patent Application No. H7-335906).